It looks like a list of functions have been formally deprecated in PHP 5.3 and may cause lots of PHP sites to crash when the server is upgraded.
Many popular shopping carts will be crashing when the server is upgraded to PHP 5.3. There is a lot of legacy code that has not been removed from a lot of popular scripts.
It does seem kind of stupid for the PHP team to generate a PHP error, rather than a warning, to let developers know that they need to upgrade the code.
Zen Cart is vulnerable, and because it is based on osCommerce, I suspect osCommerce will also crash. The Zen Cart team already has a patch set up. If you run a Zen Cart site, get the patch here:
PHP 5.3 Support - Zen Cart Support